1. Field of the Invention
The present invention relates to a power amplifier (PA). More particularly, the present invention relates to a PA having high efficiencies and better linearity.
2. Description of Related Art
The power amplifier (PA) plays an important role in a radio frequency (RF) circuit. The PA is utilized in a transmitter of the RF circuit for amplifying signals to be transmitted. RF characteristics of the PA include linearity of a power output and linearity of an output signal, which can greatly influence a performance of the entire system.
A conventional linear PA system, such as a class-A, a class-B or a class-AB PA system, biases an active device (the PA) thereof by a fixed direct current (DC), so as to obtain better linearity even though efficiency of the conventional PA is poor due to a fall-time DC power consumption. Moreover, when an input power is excessively significant, an output power thereof cannot be linearly amplified, which causes a gain reduction and a signal distortion. At this time, a signal average output power has to be reduced to maintain the linearity, which is referred to as power back-off, so that the PA cannot be operated in a high-efficiency interval, and an average efficiency of the PA is reduced.
A non-linear switching PA, such as a class-D, a class-E or a class-F PA, can theoretically reach 100% efficiency even though the linearity thereof is poor. As such, the non-linear switching PA cannot be applied to a linear modulation system. However, with a developing trend of high-speed for wireless communication systems, demands for signal linearity are increased. Therefore, although the non-linear switching PA has a better efficiency than that of the linear PA, the non-linear PA still cannot be solely applied to the high-linearity modulation system due to a poor linearity of the non-linear PA. Instead, the non-linear PA has to be utilized together with other devices to form an amplifier system for practicality.
For example, a PA system providing mode switching that is respectively disclosed in U.S. Pat. No. 6,043,707, U.S. Pat. No. 6,987,417, and U.S. Pat. No. 7,043,213 can be switched to different operation modes according to different power regions, so as to improve system efficiencies and the linearity. However, compared to the conventional linear PA, efficiency improvement of these PA systems under middle-low output power is limited.